1. Professor, College of Urban and Environmental Sciences, Peking Univ., No. 5 Yiheyuan Rd., Haidian District, Beijing, China 100871; formerly, Environmental Systems Engineering Program, Faculty of Engineering, Univ. of Regina, Regina, SK, Canada S4S 0A2 (corresponding author).
2. Professor and Canada Research Chair, Environmental Systems Engineering Program, Faculty of Engineering, Univ. of Regina, Regina, SK, Canada S4S 0A2.